1. Field of the Invention
The present invention relates to a zoom lens and a camera device including the zoom lens. The zoom lens can be a photographic lens of a digital camera, a mobile terminal, a video camera, or a silver film camera.
2. Description of the Related Art
In recent years, there have been increasing demands of various kinds for digital cameras such as high image quality, downsizing, wider angle, or higher zoom ratio. In order to achieve these, photographic lenses are required to have a zoom ratio of 5 or more, a half field of view of 38 degrees or more, generation of high-quality images adaptable to an image sensor of over 10 million pixels, downsizing, and a larger aperture.
In the prior art, there are compact-size digital cameras which realize thickness reduction of a camera case by including a reflective optical element in a lens system to bend the optical path of an incident light as well as by fixing the first lens group to prevent a change of the outer form of the camera in accordance with zooming operation.
Further, Japanese Laid-open Patent Application Publication No. 2005-215165, No. 2005-352348, No. 2007-171456, and No. 2007-3598 disclose zoom lenses each of which comprise a first lens group with a positive focal length and including a reflective optical element to bend the optical path of an incident light, a second lens group with a negative focal length, a third lens group with a positive focal length, and a fourth lens group with a negative focal length arranged in order from an object side to an image side.
However, there is a drawback in such zoom lenses having four or five lens groups with a positive-negative-positive-negative-positive reflective index distribution and a diaphragm between the second and third lens groups that to aim for a wider angle range, the first and second lens groups arranged closer to the object side than the diaphragm need be in larger size. Also, the reflective optical element in the first lens group need be increased in size.
Further, in order to improve the zoom ratio of the zoom lens, a longer optical path of incident light is needed. From camera designing point of view, the optical path should be bent in a longitudinal direction of an image sensor, which also increases the size of the reflective optical element. Moreover, with an improved zoom ratio, a distance between the reflective optical element and the diaphragm is elongated, which also leads to increasing the size of the reflective optical element. As described above, to increase wide angle range and zoom ratio with the zoom lens with such a configuration, there is a problem that a reflective optical element in a larger size is required.
In the prior art, a single negative lens is disposed on the object side of the reflective optical element for prevention of increasing the size of the reflective optical element. To achieve wider angle range and higher zoom ratio of the zoom lens, this single negative lens is required to properly correct aberration from an off-axial light beam passing through a higher position thereof at the short focus end and an axial marginal light beam passing through a higher position thereof at the long focus end. Also, the negative lens is required to have a large negative power in order to realize downsizing of the zoom lens.
With only one negative lens on the object side of the reflective optical element, it is extremely difficult to achieve a large negative power and proper aberration correction of the off-axial light beam and the axial marginal light beam at the same time, as well as increase in the zoom ratio and half the field of view.
Japanese Laid-open Patent Application Publication No. 2005-215165 discloses a variable zoom lens with a high zoom ratio of 7 or more. However, it fails to realize a wide angle range with half the field of view of 38 degrees or more at short focus end. Japanese Laid-open Patent Application Publication No. 2005-352348 discloses a wide angle, high zoom ratio zoom lens with a zoom ratio of about 35 and half the field of view of 38 degrees or more, however, it is not compact enough in size and has a very large distortion so that it cannot achieve good optical performance. Japanese Laid-open Patent Application Publication No. 2007-171456 discloses a variable zoom lens with a zoom ratio of 7 and half the field of view of 38 degrees or more at short focus end. However, it causes a difficulty in camera layout since a reflective optical element is configured to bend an optical path of incident light in a short side direction of an image sensor. A zoom lens disclosed in Japanese Laid-open Patent Application Publication No. 2007-3598 cannot achieve half the field of view of 38 degrees or more and zoom ratio of 5 or more at the short focus end.